1. Field of the Invention
The present invention relates to an optical system for collimating an elliptical light beam, and particularly to an optical system for efficiently collimating elliptical light beams emitted from a side light emitting laser diode and an optical device using the same.
2. Related Art
Optical disks are widely used data store media, and are being developed to store more information than previously. Since higher data storing density is demanded of optical disks, optical disk reading/writing systems correspondingly need to be more precise and sophisticated.
Referring to FIG. 1, a conventional optical device 10 for providing a collimated parallel round light beam for reading/writing to an optical disk is shown. The optical device 10 includes a light source 110, an optical holographic element 120, a round collimating lens 130, a reflective mirror 140, an object mirror 150, and an optoelectronic detector 160. In operation, the light source 110 provides a light beam of a certain wavelength. The light beam reaches and passes through the optical holographic element 120, and thereafter is collimated by the collimating lens 130 into a parallel light beam. The parallel light beam is then reflected by the reflective mirror 140 to the object mirror 150. The object mirror 150 converges the parallel light beam to a recording layer 170 of an optical disk (not labeled). The light beam converged to the recording layer 170 is modulated in accordance with the data recorded thereon or written thereon, and is then reflected by the optical disk back to the object mirror 150. The light is then transmitted back to the optical holographic element 120 along the above-described input path. The optical holographic element 120 is adapted for deviating light beams that pass therethrough in the return direction. Therefore, the light beam is transmitted to and detected by the optoelectronic detector 160, rather than being transmitted to the light source 110. According to the light beam received, the optoelectronic detector 160 outputs an electronic signal, from which the information recorded on or written to the optical disk can be interpreted or identified.
A typical optical system adopts a side light emitting laser diode as a light source. Referring to FIG. 2, such a side light emitting laser diode 9 has a rectangular waveguide type resonation cavity. The laser light beam emitted from the resonation cavity has different diverging angles in horizontal directions and vertical directions respectively, and thus provides an elliptical light beam. Typically, the horizontal diverging angle is about ±10° and the vertical diverging angle is about ±30°. An elliptical light beam has to be intercepted or converted to a round light beam for use in the optical system.
In the above-described optical device 10, the round collimating lens 130 is employed for intercepting a round core part of the elliptical light beam and thus obtaining a round light beam. The collimating lens 130 generally has a diameter shorter than a corresponding short (e.g., horizontal) axis of a light spot projected by the elliptical light beam incident thereon. The core part of the elliptical light beam is allowed to pass through the round collimating lens 130, and the peripheral part of the elliptical light beam is dissipated. Referring to FIG. 3, this is a graph of a relationship between diverging angles of the elliptical light beam output by the side light emitting laser diode (X-axis) and intensity of light output by the collimating lens 130 (Y-axis). Various different horizontal diverging angles are collectively shown as the line θH, and various different vertical diverging angles are collectively shown as the line θv. The space between any two horizontally opposite points on the line θH represents the round core part of the elliptical light beam that is intercepted by the round collimating lens 130. The horizontal space between each such point and the corresponding point on the line θv represents a peripheral part of the elliptical light beam that is dissipated. As seen in FIG. 3, even if the round collimating lens 130 intercepts the elliptical light beam with a minimal amount of loss of light intensity (i.e. when both of the diverging angles are small), the amount of loss of light intensity is still quite large. Therefore, in general, a side light emitting laser diode with high power is needed to compensate for the loss of light intensity. However, high-power laser diodes are not only more costly, but also consume more power.
Therefore, what is needed is an optical system for efficiently collimating an elliptical light beam.